Ultrahigh quality factor microresonators have extremely long photon lifetimes, enabling high circulating power. As a result of the amplification of the input optical power, these devices are able to excite various nonlinear optical phenomena, such as four-wave mixing (FWM) and stimulated Raman scattering (SRS). Previously, FWM and their cascaded peaks enabled frequency comb generation in silica toroidal resonators. However, high input power (> 60 mW) is required to generate broad frequency combs (>500 nm span) due to the intrinsic material properties of silica.
In this present work, we modify the material properties of silica by coating a silica toroidal cavity with a thin film of Zirconium (Zr) doped solgel. This thin layer substantially improves the performance of Raman-Kerr frequency comb generation in hybrid microcavities. A series of concentrations of Zr-doped solgel are synthesized, and the effects of the Zr dopants are characterized with both theoretical calculations and experimental measurements. Doping Zr into the silica matrix enables the Zr-doped devices to have a lower dispersion than a bare silica device, enabling the frequency comb span to increase. Additionally, Zr dopants increase the efficiency of the SRS process. As Zr concentrations increase, Stokes as well as anti-Stokes Raman scattering and their cascaded FWM peaks start contributing to the formation of the frequency comb, generating Raman-Kerr frequency combs. Consequently, Zr doping enables large frequency comb spans with significantly reduced input power.
As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators fabricated from silica have demonstrated extremely low threshold Raman lasing behavior. However, the efficiency of the lasing has been poor due to the intrinsic low Raman gain of silica (~5%). By grafting oriented monolayers of highly nonlinear organic small molecules to the surface of conventional silica whispering gallery mode optical resonators, we demonstrate a new strategy for fabricating Raman lasers. The laser efficiency is improved from 4% to over 40%. Density functional theory is performed to understand the mechanism giving rise to the improvement. This chemistry-based approach could be applied to nearly any whispering gallery mode cavity geometry to improve performance, providing a universal strategy for device performance improvement.
Whispering gallery mode optical resonators integrated on silicon have demonstrated low threshold Raman lasers. One of the primary reasons for their success is their ultra-high quality factors (Q) which result in an amplification of the circulating optical field. Therefore, to date, the key research focus has been on maintaining high Q factors, as that determines the lasing threshold and linewidth. However, equally important criteria are lasing efficiency and wavelength. These parameters are governed by the material, not the cavity Q. Therefore, to fully address this challenge, it is necessary to develop new materials. We have synthesized a suite of metal-doped silica and small molecules to enable the development of higher performance Raman lasers. The efficiencies and thresholds of many of these devices surpass the previous work. Specifically, the silica sol-gel lasers are doped with metal nanoparticles (eg Ti, Zr) and are fabricated using conventional micro/nanofabrication methods. The intercalation of the metal in the silica matrix increases the silica Raman gain coefficient by changing the polarizability of the material. We have also made a new suite of small molecules that intrinsically have increased Raman gain values. By grafting the materials to the device surface, the overall Raman gain of the device is increased. These approaches enable two different strategies of improving the Raman efficiency and threshold of microcavity-based lasers.
As a result of their ability to amplify input light, ultra-high quality factor (Q) whispering gallery mode optical resonators have found numerous applications spanning from basic science through applied technology. Because the Q is critical to the device’s utility, an ever-present challenge revolves around maintaining the Q factor over long timescales in ambient environments. The counter-approach is to increase the nonlinear coefficient of relevance to compensate for Q degradation. In the present work, we strive to accomplish both, in parallel. For example, one of the primary routes for Q degradation in silica cavities is the formation of water monolayers. By changing the surface functional groups, we can inhibit this process, thus stabilizing the Q above 100 million in ambient environments. In parallel, using a machine learning strategy, we have intelligently designed, synthesized, and verified the next generation of small molecules to enable ultra-low threshold and high efficiency Raman lasing. The molecules are verified using the silica microcavity as a testbed cavity. However, the fundamental design strategy is translatable to other whispering gallery mode cavities.
Whispering Gallery Mode (WGM) silica microresonators are a particularly unique group of microcavities in the sense that they can confine light inside the device for an extended period of time while maintaining a high quality (Q) factor due to the total internal reflection. As a result, WGM resonators have high circulating optical power, which can cause nonlinear optical processes such as stimulated Raman scattering (SRS). It has been demonstrated that SRS has been observed in various WGM silica microresonators with the sub-mW Raman lasing threshold. However, in case of the Raman lasing efficiency, it is limited by the intrinsic property of silica itself, which is the Raman gain coefficient. Therefore, in the present work, we introduce a hybrid silica toroidal microcavity in order to enhance the Raman lasing efficiency. First, we synthesize a suite of silica sol-gels doped with a range of Zirconium (Zr) concentrations and integrate the material with silica toroidal microresonator. The intrinsic Raman gain of the Zr-doped silica is measured using Raman spectroscopy, and the values show a clear dependence on Zr dopant concentrations. The lasing performance is characterized using a 765 nm pump source, and the Raman emissions for the coated devices are detected at 790 nm and longer. The lasing emission and characteristic threshold curves are quantified using both an optical spectrum analyzer and an optical spectrograph. The lasing slope efficiency of exhibits a marked increase from 3.37% to 47.43% as the Zr concentration increases due to the Raman gain improvement. These values are particularly notable as they are the unidirectional, not bidirectional, lasing efficiencies.
High and ultra-quality factor (Q) optical resonators have been used in numerous applications, ranging from biodetection and gyroscopes to nonlinear optics. In the majority of the measurements, the fundamental optical mode is used as it is easy to predict its behavior and subsequent response. However, there are numerous other modes which could give improved performance or offer alternative measurement opportunities. For example, by using a mode located farther from the device surface, the optical field becomes less susceptible to changes in the environment. However, selectively exciting a pre-determined, non-fundamental mode or, alternatively, creating a “designer” mode which has one’s ideal properties is extremely challenging. One approach which will be presented is based on engineering a gradient refractive index (GRIN) cavity. We use a silica ultra-high-Q toroidal cavity as a starting platform device. On top of this structure, we can controllably deposit, layer or grow different materials of different refractive indices, with nm-scale precision, creating resonators with a GRIN region co-located with the optical field. Slight adjustments in the thicknesses or indices of the films result in large changes in the mode which is most easily excited. Even in this architected structure, we have maintained Q>1 million. Using this approach, we have demonstrated the ability to tune the properties of the device. For example, we have changed the thermal response and the UV response of a device by over an order of magnitude.